PATTERSON — Farmers across the country are rallying to fight a Republican-sponsored bill that would force them and all other employers to verify the legal immigration status of their workers, something they say could imperil not only future harvests but also the agricultural community’s traditional support for conservative candidates.

The bill was proposed by Rep. Lamar Smith, R-Texas, the chairman of the House Judiciary Committee. It would require farmers — who have long relied on a labor force of immigrants, a majority here without legal documents — to check all new hires through E-Verify, a federal database run by the Department of Homeland Security devised to ferret out illegal immigrants.

Farm laborers, required like other workers to show that they are authorized to take jobs in the United States, often present Social Security numbers and some form of picture ID. Employers, many of them labor contractors providing crews to farms, have not been required to check the information and are discouraged by antidiscrimination laws from looking at it too closely. But it is an open secret that many farmworkers’ documents are false.

Supporters of E-Verify, an electronic system that is mandatory for most federal contractors but voluntary for other employers, argue that it would eliminate any doubt about workers’ legal status. But farmers say it could cripple a $390 billion industry that relies on hundreds of thousands of willing, low-wage immigrant workers to pick, sort and package everything from avocados to zucchini.

“This would be an emergency, a dire, dire situation,” said Nancy Foster, president of the U.S. Apple Association, adding that the prospect of an E-Verify check would most likely mean that many immigrant workers would simply not show up. “We will end up closing down.”

That sentiment is echoed by growers like George Bonacich, an 81-year-old apricot farmer who has been working the same patch of land in Patterson, 80 miles east of San Francisco, since 1969.

This year, Bonacich employed up to 100 farmhands to pick a total of 50 to 100 tons each day, often in triple-digit heat. He speaks passionately about his employees — “They’re good people, hard-working,” he said — and plainly about what would happen if E-Verify were to become the law of the farmland.

“If we don’t have enough labor at peak time, the fruit goes on the ground,” he said. “The fruit will only stay on the tree so long.”

While Smith’s bill seems to have a good chance of passing the House, the Senate, controlled by Democrats, appears more skeptical. Democrats have said they will point to a Congressional Budget Office report on a similar bill that concluded it would cost the federal government $22 billion over a decade, from lost tax revenues now collected from the paychecks of illegal immigrants ineligible for services.

In a May letter to the members of the Judiciary Committee, Bob Stallman, president of the American Farm Bureau, cited a Labor Department survey placing the percentage of undocumented workers in the fields at more than 50 percent. Other groups say the figure is closer to 70 percent. Denying farmers that labor supply, Stallman wrote, would cost them $5 billion to $9 billion annually.

Smith’s bill has attracted more solid support from nonagricultural business leaders, opening a divide between them and agricultural interests. Many nonfarm businesses have concluded that some form of employee verification is inevitable. National organizations of restaurant owners and home builders gave their backing. The San Antonio Hispanic Chamber of Commerce, which unites Latino businesses in Smith’s district that have often been at odds with him, is leaning toward endorsing the bill, said Ramiro A. Cavazos, president of the chamber.

Still, Smith, whose district includes parts of suburban San Antonio and Austin as well as a large part of the nearby Hill Country, recently acknowledged the surge of worry in rural areas. He said he would soon introduce a separate bill to “address the needs of the agriculture industry,” either proposing changes to the current federal temporary farm worker program, known as H-2A, or offering a new guest worker program.

Smith’s E-Verify bill also includes a three-year grace period before growers would have to comply. But such caveats have done little to quell opposition from farm groups, who have been pleading for years for an overhaul to allow a legal immigrant work force.

And that discontent could manifest itself in elections, farm representatives warn.

“Most of our folks are Republicans,” said Paul Wenger, president of the California Farm Bureau. “But if the Republicans do this to them without a workable worker program, it will change their voting patterns or at very least their involvement in politics.”

Smith said requiring employers to determine that new hires were eligible to work would prevent illegal immigrants from taking jobs at a time of high joblessness for Americans. “We could open up millions of jobs for unemployed Americans and legal workers,” he said.

But farmers and their advocates scoff at that notion, saying that regardless of high unemployment, few American workers are willing to sign up for what are often hard, hot and long hours in the fields.

“People just don’t want to do farm work,” Wenger said. “They don’t want to pick berries. They don’t want to pick lettuce. And the pay is just as good as working at the hamburger shop or making up hotel rooms, but they just don’t want to do the work.”

Mike Carlton, director of labor relations for the Florida Fruit and Vegetables Association, agreed. He said his group monitored hiring by citrus growers, who are required to offer jobs to Americans before they can turn to the H-2A program for temporary foreign laborers.

In one sample, Carlton said, 344 Americans came forward to fill 1,800 pickers’ jobs; only eight were still working at the end of the two-month season.

Carlton said Florida growers had flocked to Washington, telling lawmakers they had glimpsed the possible impact of Smith’s proposal after a verification mandate narrowly failed in the Florida Legislature this spring. “Just the prospect of it, and some of our workers left the state,” Carlton said.

Labor shortages were also reported by Georgia growers, said Charles Hall, executive director of the Georgia Fruit and Vegetable Growers Association, after lawmakers there imposed a state mandate to use E-Verify this spring.

Some of the most forceful feedback has come from Smith’s home state, where farmers have the backing of the larger Texas Business Association.

The group used its clout recently to kill an immigration crackdown law in the State Legislature, even though it was supported by Gov. Rick Perry, a possible Republican presidential contender.

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